The invention relates generally to power steering systems and, in particular, to methods and apparatuses for electronically controlled power steering systems.
Power steering systems are used in motor vehicle applications to augment the steering effort provided by the driver of a vehicle. Many conventional systems employ a four-way open-center "rotary" control valve having "follow along" position feedback in which road feel is artificially induced by deflection of a torsion bar. Other power steering systems typically employ a four-way open-center "reaction" control valve also having "follow along" position feedback wherein road feel is induced directly via hydraulic reaction forces.
Conventionally, the control valves of the power steering systems are mechanically coupled to the vehicle's steering shaft and are typically located just ahead of the vehicles's steering gear assembly. Unfortunately, this placement often results in significantly compromising the static and dynamic performance of the host power steering systems. For example, such power steering systems may generate poor response during lower steering wheel torque inputs and/or generate excessive response to small changes in the steering wheel torque input at higher levels thereof. Dynamically, some conventional power steering systems are also subject to sluggish behavior which produces a response that is delayed beyond normally anticipated human perception times. In addition, some power steering systems are subject to undesirably rapid steering wheel motion without a concomitant increase in required steering wheel torque. In other words, the steering force output of such systems cannot be said to be rate stable.
Further, power steering systems have been recently modified to include "speed sensitive" steering. In general, most "speed sensitive" systems increase steering forces at high vehicular speeds by reducing hydraulic fluid flow through the control valves in order to reduce valve gain. An alternative power steering system, known as a "Variable-Assist" Power Steering System is described in SAE Technical Paper No. 880707 entitled 1988 LINCOLN CONTINENTAL VARIABLE-ASSIST POWER STEERING SYSTEM by J. J. Duffy.
In order to comprehend system performance characteristics required for an "ideal" power steering system, it is first necessary to understand the nature of a vehicle's overall information (i.e., feedback) inputs are visual and tactile. The driver senses the vehicle's overall position and dynamic attitude. The driver then compares the vehicle's actual lateral position and yaw, as well as any lateral acceleration and roll motion, with those he desires and mentally formulates error signals representative of the differences there between. The driver's response is to continuously modify his input to the vehicle's steering wheel. Typically, a driver completes the derived response by applying a predetermined level of torque to the steering wheel in anticipation of it executing a predetermined degree of rotational motion. If both the steering wheel and the vehicle behave precisely as anticipated, the steering system can be said to be "ideal".
Typical "rotary valve" equipped power steering systems have substantially nonlinear static performance characteristics. Unexpectedly small changes in the output force are concomitant with given changes in steering wheel torque at low output force levels while unexpectedly large changes of output force are concomitant with given changes in steering wheel torque at high output force levels. Both conditions can result in excessive steering wheel motion with respect to the predetermined amount of rotational motion anticipated by the driver. In the low output force condition, rotational compliance inherent in the rotary valve results in relatively large steering wheel motions before the required output force levels are obtained. And, in the high output force condition, lack of any apparent tactile input torque modulation often results in over-correction of the steering wheel's position.
In many cases, rotary valve equipped power steering systems also produce relatively slow response at low output force levels. This is indirectly the result of the method employed for achieving overall closed-loop stability of the system. All power steering systems are closed-loop servo systems which must achieve unity gain cross-over in a satisfactory manner for system stability. (A thorough discussion of servo system stability is contained in a book entitled FEEDBACK AND CONTROL SYSTEMS by Di Stefano III, Stubberud and Williams, and published by the McGraw-Hill Book Company as one of Schaum's Outline Series.)
Most rotary valve equipped power steering systems manufactured today utilize an O-ring "brake" to disrupt normal system roll-off characteristics and to preclude system oscillation Such an O-ring can be seen quite clearly in FIG. 2 of U.S. Pat. No. 4,452,274 (Haga et al). The O-ring is located in a groove in the steering shaft which is typically ported to incoming hydraulic fluid pressure such that the O-ring acts as a brake between the steering shaft and the valve sleeve whenever the incoming hydraulic fluid pressure is elevated.
The result of poor response at low steering forces is that the driver of a vehicle having rotary valve power steering is unable to exert "fine" direction control over the vehicle under transient conditions such as ruts in the road or side wind gusts. Vehicles having the slow response time noted above typically have a wide range of lateral position uncertainty under the most severe transient conditions which is generally greater than the range of lateral position uncertainty for vehicles equipped with backlash free manual steering systems.
To most modern vehicle owners, manual steering systems are deemed undesirable because they provide no steering assist. However, exclusive of their performance superiority over rotary valve systems, both manual and "reaction valve" equipped power steering systems have shortcomings when measured against the "ideal" standards outlined above. This is because the driver tactilely expects higher input torque requirements when entering a turn at a higher vehicular speed. Exclusive of exciting tire slip angles and the like, this is simply not the case with any known steering system unless it is supplemented with a shock absorber-type damper linked to the vehicle's dirigible wheels. In fact, under certain nonlinear conditions involving tire slip angles or other transient conditions, rapid movements of the steering wheel can even result in lower input torques. The reason for this unstable behavior is that known steering systems, which do not feature the above noted shock absorber-type damper, have minimal rate stabilization, or feedback, inherent in their operation.
In accordance with the teachings of the present invention, the concept of utilizing "rate" feedback to enhance steering stability can be extended to form the basis for an improved type of speed sensitive steering known herein as "rate derived-speed sensitive" steering. As detailed hereinbefore, conventional "speed sensitive" steering systems merely increase steering force (i.e., magnitude derived speed sensitive steering systems). To promote superior stability, it is preferable to implement a "rate derived-speed sensitive" steering system wherein rate feedback is increased as a function of vehicular speed. Therefore, tactile resistance is felt by the driver in direct proportion to the applied steering wheel rotational velocity. In addition, the tactile resistance would increase as a selected function of vehicular speed.
Accordingly, the present invention includes various structural embodiments and methods for an electronically controlled power steering system wherein the advantages associated with the "ideal" steering system, explained above, are substantially realized. In a first embodiment, a torque transducer associated with the steering shaft is used to provide a signal indicative of steering wheel torque. This applied torque derived, signal (hereinafter "torque signal") is suitably frequency compensated or manipulated and amplified The compensated and amplified torque signal is then utilized to provide an "output signal" which drives a torque motor which, in turn, actuates a rotary control valve. The rotary valve then provides differential fluid flow to a power cylinder to provide steering assist. If desired, the signal amplification can be varied as a function of vehicular speed to implement "magnitude derived-speed sensitive" steering.
The torque motor driven rotary valve can be located remotely from the steering shaft. Preferably, it is located within a pump assembly utilized to provide hydraulic fluid to the power steering system. Fluid passage to-and-from the rotary valve is accomplished via direct porting with hydraulic lines used for transmitting fluid to either end of the power cylinder.
Functionally, nonlinear amplification of the torque signal is used to increase system gain at low input torque levels and reduce system gain at high input torque levels. Thus, the static performance characteristic of the overall power steering system is modified such that it is much more nearly ideal. The frequency compensation is utilized to ensure stable system operation together with an acceptable unity gain cross-over frequency. Thus, the system response is tactilely acceptable in both magnitude and response time under virtually all normal and transient conditions.
The nonlinear amplification of the torque signal can be accomplished in a continuous manner through zero values of applied torque. Alternatively, the torque signal can be initialized at selected left and right values of applied torque. This provides a "dead zone" near the zero values of applied torque wherein no steering assist is provided. If this is done, the effect is to emulate power steering systems wherein components of the control valve assembly are physically locked or "intermeshed" together at low values of applied torque. In this manner, unassisted manual steering is present at low values of steering load.
According to a second embodiment, a tachometer is located within the steering unit. The tachometer is used to provide an additional signal indicative of steering rate (hereinafter "rate signal"). The "rate signal" is suitably frequency compensated and amplified. The amplification can be accomplished linearly or it can be accomplished nonlinearly as a function of its own magnitude and/or the magnitude of the torque signal. The compensated and amplified rate signal is then utilized to selectively modify the "output" signal applied to the torque motor in order to provide the "rate feedback" mentioned hereinbefore. Also, the amplification can be varied as a function of vehicular speed to implement the "rate derived-speed sensitive" steering of the present invention.
In a third preferred embodiment, pressure transducers are utilized to measure differential pressure applied to the power cylinder. Output signals from the pressure transducers are utilized to provide a "pressure" signal which, in turn, is feedback and selectively compared with the "torque" signal. Such feedback ensures that the differential pressure applied to the power cylinder is actually the desired function of the torque applied to the steering wheel. Thus, while the power steering system described in the third embodiment includes application of a rotary valve, it emulates the superior operational characteristics of power steering systems utilizing reaction valves.
In a fourth embodiment, a reaction valve is substituted for the rotary valve. An exemplary reaction valve suitable for this purpose is one that was first described in U.S. Pat. No. 4,922,803, entitled FOUR-WAY VALVE issued May 8, 1990 to Edward H. Phillips, and as modified in U.S. Ser. No. 485,637, entitled VARIABLE RATIO REACTION VALVE by Edward H. Phillips, filed Feb. 23, 1990, both of which are assigned to the common assignee of the present application and both of which are incorporated by reference herein. These control valves are known as "variable ratio" reaction valves and have the advantage that their static performance characteristics are selectively chosen in a manner that substantially emulates the ideal static performance characteristics described above. Thus, the torque signal can be linearly amplified rather than nonlinearly amplified in forming the "output" signal used to drive the torque motor.
The control valve of the fourth embodiment is superior over conventional systems in that it is a reaction valve such that the torque imposed on the torque motor is a more accurate model of the actual steering load. Since the torque delivered by the torque motor is substantially a linear function of the output signal and the torque signal and applied torque are also linearly related thereto, the applied torque is likewise a more accurate model of the actual steering load. As will be explained hereinafter, this characteristic aids in centering the steering wheel after a turn. In addition, since reverse differential pressure generated by the mechanical backup steering can exert reverse torque on the reaction valve. This tends to open the reaction valve so that mechanical backup steering can be accomplished with reduced effort. Thus, the fourth preferred embodiment of the present invention substantially emulates the superior "ideal" power steering system as outlined hereinbefore.
Because the fourth embodiment includes "rate" feedback, it is possible to eliminate a pair of controlled orifice flow restrictors commonly utilized with reaction valves as described in U.S. Pat. No. 4,922,803 (See FIGS. 22-26C). This is possible because the hydraulically-derived rate stabilization provided by the controlled orifice flow restrictors of U.S. Pat. No. 4,922,803 is electronically implemented in the present invention by the rate feedback function. This has the further advantage of eliminating hydraulic power loss associated with the controlled orifice flow restrictors. Thus less hydraulic power is required whenever the steering system is in motion.
As in all power steering systems, a fail-safe operation is required for all of the various embodiments of the present invention. In the case of the first, second and fourth preferred embodiments of the present invention, current passing through the torque motor is measured and compared with calculated current values. In the case of the third preferred embodiment of the present invention, the output signals from the pressure transducers are compared with calculated differential values thereof. In either case, excessive variation thereby causes current to stop flowing through the torque motor.
Various other objects and advantages of the present invention will become more apparent to one skilled in the art from reading the following specification taken in conjunction with the appended claims and the following drawings.